Don't bother with .451 inch balls. Get the .454 balls. I think you'll find them more accurate and they will certainly seal much better.
I've given up on .451 inch balls. The .454 balls ensure that I have a good seal and, because swaging them into the chamber creates a wider bearing surface for the rifling to grip, they seem more accurate.
You may also use .457 inch balls in your Remington repro, though the .457 ball is intended for the Ruger Old Army. You won't hurt a thing by using .457 inch balls, but you will find it more difficult to seat the ball, so ensure you leave ample room for it to be seated flush.
For a great deal of information, scroll down to my earlier post, "How to Best Use a Cap and Ball Revolver." You'll find much of what you need to know in that post, based on 35 years of shooting cap and ball revolvers.